Efforts to improve the environmental situation around leather factories are very much concerned with reducing the chromium content of the wastewater. In addition to the recovery of unused chromium from residual tanning and retanning liquors, high-extraction chrome tanning processes have been adopted with a view to achieving this objective (K. Faber, Bibliothek des Leders, Vol. 3, pages 158 et seq, UmschauVerlag, Frankfurt/Main, 1st Edition, 1985). In Faber excesses of chromium which are not actually required and which always have to be circulated are not used for tanning as they are in recycling processes. Instead, only that quantity of chromium which is needed to establish the desired chromium content in the leather is used for tanning. Accordingly, the residual tanning and retaining liquors contain only small quantities of chromium.
To improve the extraction of chromium, it has been proposed, inter alia, to use so-called crosslinking aliphatic dicarboxylic acids, such as for example succinic acid, adipic acid and glutaric acid, for pickling or for chrome tanning (see for example Das Leder 23 (1972), 174 et seq; Das Leder 28 (1977), 155 et seq; Leder- und Hautemarkt 30 (1978), 132 et seq).
The aliphatic dicarboxylic acids are added to the chrome tanning liquor after the chrome tanning material in the form of their disodium salts. Although the extraction of chromium is distinctly improved in relation to a conventional tanning process, the quality of the leather can nevertheless be adversely affected (cf. K. Faber, loc. cit., page 83). The distribution of chromium and dyeability are impaired, particularly with thick or unskived hides. Another negative factor is that, in the described process, the penetration of the chrome tanning agent through the skin cross-section is impeded by the presence of the dicarboxylic acids (Leder- und Hautemarkt 30 (1978) 140).
To improve the situation, it is recommended to work at final pH values in the tanning liquor below 4.0 and at final temperatures of around 40.degree. C. However, this only provides for extraction levels of up to 95% (Das Leder 28 (1977), 157).
In addition, it is recommended to add the dicarboxylic acid salts in several portions. However, this is an additional complication of the chrome tanning process.
The difficulties described above are eliminated by the process described in German 2,424,301, in which tanning mixtures consisting of chromium(III) salts, acid-binding agents and aliphatic C.sub.4 -C.sub.6 dicarboxylic acids or salts thereof are initially used together with conventional chrome tanning agents for chrome tanning. To obtain high extraction, at least 1.6 mol dicarboxylic acid has to be used per mol chromium oxide in the tanning mixture for full tanning.
In addition, it is known that solutions of chromium(III) salts and sodium adipate can be used for the chrome tanning of calf skins (Journal of the International Society of Leather Trades Chemists 27 (1943), 83 et seq). In this case, however, more than 1.5% chromium oxide (based on pickle weight, corresponds to considerably more than 1.5% chromium oxide, based on skin weight) has to be used to obtain an adequate tanning effect (boil dressing). The chrome liquors used have a chromium oxide content of at most only 2.8% and are therefore unsuitable for use on a commercial scale. Where more than 1.5% chromium oxide (based on pickle weight) is available, the extraction of chromium is distinctly in excess of 1.0 g chromium oxide/l.
Accordingly, in all hitherto known high-extraction chrome tanning processes where aliphatic dicarboxylic acids are co-used, the total quantity of chrome tanning material and the dicarboxylic acids always have to be added to the liquor in unreacted form in at least two portions.